Lixiviation is a technique used to extract a soluble component from a solid mixture by washing or percolation, i.e. leaching. World-wide present practice for extracting precious metals by lixiviation is carried out using cyanide solutions, mainly sodium cyanide. Because cyanides are so highly toxic, and because they cause substantial environmental problems, the use of cyanides is now falling into disfavor. Moreover, cyanides are costly materials which makes their use economically disadvantageous. Moreover, the use of cyanide solutions is at best difficult and at worst impossible with respect to some ores, especially those containing copper and/or manganese, since these materials easily contaminate the cyanide; and such materials are frequently present to the extent that poor recoveries of the precious metals are obtained using cyanide solutions.
Indeed, with respect to the last problem mentioned immediately above, there are many difficult-to-treat ores in existence which contain manganese and significant quantities of silver and/or gold, and from which it would be desirable to extract these precious metals, and if a suitable and sufficiently inexpensive technique existed for such recovery. However, present techniques are simply not adequate and these ores remain an untapped mineral resource.
Copper-bearing sulphur-containing ores, such as chalcopyrite, often contain small quantities of gold and silver which, desirably, should be recovered. Although the problem of recovering such precious metals, as well as the copper, has received considerable attention in the past, much of the work carried out in this connection, insofar as commerical processing is concerned, has involved the recovery of precious metals using pyrometallurgical processes for the recovery of the copper.
One attempt to solve the above identified problems is disclosed in the Genik-Sass-Berecowsky et al U.S. Pat. No. 4,070,182. This patent proposes the use of ammonium thiosulfate as a secondary leach for the recovery of silver and gold, in conjunction with a hydrometallurgical process for the recovery of copper from the copper-bearing sulphidic ore. FIG. 3 of this patent shows a flow diagram for the extraction of precious metals from chalcopyrite concentrate before the main leaching step for extraction of copper. However, this patent appears to provide no instruction as to how to maintain the thiosulphate radical stable, and does not even appear to recognize the problem of thiosulphate instability. This patent also does not clearly teach the necessity of maintaining an alkaline pH in the thiosulfate leach liquor when starting with a raw ore, although the need for an alkaline pH is mentioned in conjunction with thiosulfate extraction following a copper recovery leach. Furthermore, this patent provides no guidance with respect to the extraction of precious metals from difficult ores containing manganese.